Not just for YC folks - Microsoft has their BizSpark program, which is pretty awesome for startups - it gives you access to basically every Microsoft product ever made for free.
I can confirm that this program is great! Our startup leveraged it as much as we could. It also includes other free things such as Windows Azure hours.
We are using BizSpark for two months now and it helps a lot, but it's not for free you have to pay 100$ after three years.